Balanced mixotrophy method

ABSTRACT

Methods of culturing mixotrophic microorganisms in a state of balanced gas composition are disclosed. Parameters of a culture of mixotrophic microorganisms may be controlled to reduce the requirements of externally supplied gases and optimize the production and consumption of gases within the culture by the phototrophic and heterotrophic metabolisms of the mixotrophic microorganisms.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/US2013/069047, filed Nov. 8, 2013 entitled Balanced Mixotrophy Methods, which claims benefit of U.S. Provisional Application No. 61/724,710, filed Nov. 9, 2012, entitled Methods of Culturing Microorganisms in Mixotrophic Conditions; U.S. Provisional Application No. 61/798,969, filed Mar. 15, 2013, entitled Mixotrophy Systems and Methods; and U.S. Provisional Application No. 61/891,990, filed Oct. 17, 2013, entitled Balanced Mixotrophy Methods, the entire contents of which are hereby incorporated by reference herein.

BACKGROUND

Mixotrophic microorganism, such as microalgae and cyanobacteria, may be cultivated to produce proteins, lipids, carbohydrates, pigments, and polysaccharides that are useful in a variety of products. The metabolism of mixotrophic microorganisms includes both phototrophic and heterotrophic metabolisms to utilize multiple inputs as energy and carbon for growth and production of gases. When light is utilized as an energy source and inorganic carbon (e.g., carbon dioxide) is utilized as a carbon source by the phototrophic metabolism for growth, the microorganism's net oxygen production is positive and carbon dioxide is consumed. When an organic carbon source is utilized as both an energy source and a carbon source by the heterotrophic metabolism for growth, the microorganism's net carbon dioxide production is positive and oxygen is consumed. With the production and consumption of oxygen and carbon dioxide alternating or happening simultaneously within a mixotrophic microorganism, the composition of gasses saturated in the culture medium is continually changing.

Using conventional air sparging and carbon dioxide injection in a mixotrophic culture does not maintain a correct balance of gases for efficient growth in a continually changing culture. The gas imbalance created by conventional air sparging and carbon dioxide injection in a mixotrophic culture may result in gases being lost to the atmosphere before utilization by the microorganisms in the aqueous medium, which reduces the method efficiency and wastes resources. Toxic saturation of a gas in the culture medium may also occur, which may provide an inhibiting effect on the growth of the mixotrophic microorganisms. The use of air sparging may also create foam (comprised of proteins, polysaccharides, carbon and other organics), which can facilitate the proliferation of contaminating organisms (e.g., bacteria, fungi and predators) by harboring the contaminating organisms and providing a feed source for the contaminating organisms. The foam may also block or limit light available for photosynthesis. Both the facilitation of contaminating organisms and blocking of light may inhibit the growth of the mixotrophic microorganisms. Another drawback of systems using conventional air sparging and carbon dioxide injection is the need to supply gasses from an external source to bioreactors, which may reduce the length of each run of a bioreactor circulation path or require additional gas injection points, thus potentially limiting the volumetric capacity of the bioreactor. The additional gas injection points may also increase the costs of constructing and operating the bioreactor.

Therefore, there is a need in the art for a method of culturing mixotrophic microorganisms which efficiently utilizes carbon dioxide and oxygen, and reduces the accumulation of foam in the culture.

SUMMARY

Embodiments described herein relate generally to methods for culturing mixotrophic microorganisms in a state of balanced gas composition. Parameters of a culture of mixotrophic microorganisms may be controlled to reduce the requirements of externally supplied gases and optimize the production and consumption of gases within the culture by the phototrophic and heterotrophic metabolisms of the mixotrophic microorganisms.

In some embodiments of the invention, a method of growing mixotrophic culture of microorganisms comprises: circulating a culture of mixotrophic microorganisms in an aqueous culture medium through a circulation path of a bioreactor system comprising at least one light source supplying light and at least one organic carbon supply device supplying organic carbon to the culture of mixotrophic microorganisms; detecting at least one parameter of the culture of mixotrophic microorganisms selected from the group consisting of: pH, dissolved oxygen, and dissolved carbon dioxide, with at least one sensor; transmitting data from the at least one sensor to a programmable logic control system (PLC), wherein the PLC controls the supply of at least one selected from the group consisting of light and organic carbon to the culture of mixotrophic microorganisms; and continuously adjusting the supply of at least one selected from the group consisting of light and organic carbon to the culture through the PLC based on the sensor data to maintain the at least one selected from the group consisting of the pH, dissolved oxygen, and dissolved carbon dioxide levels of the culture at a predetermined threshold level.

In some embodiments, the organic carbon is supplied to the culture when the dissolved oxygen level is above a predetermined threshold level. In some embodiments, the predetermined threshold level of dissolved oxygen is between 25-200% of the saturation level of the aqueous culture. In some embodiments, an acidic organic carbon is supplied to the culture when the pH level is above the predetermined threshold level. In some embodiments, a basic organic carbon is administered to the culture when the pH level is below the predetermined threshold level. In some embodiments, the predetermined threshold pH level is between 6 and 9. In some embodiments, the predetermined threshold pH level is between 1 and 5. In some embodiments, the organic carbon is supplied to the culture when the dissolved carbon dioxide level is below the predetermined threshold level.

In some embodiments, the method further comprises supplying gas to the culture of mixotrophic microorganisms with at least one gas supply device, and wherein the PLC controls the supply of gas. In some embodiments, the gas may comprise carbon dioxide, oxygen, air, or nitrogen. In some embodiments, the carbon dioxide is supplied to the culture when the dissolved oxygen level is below the predetermined threshold level. In some embodiments, the carbon dioxide is supplied to the culture when the pH level is above the predetermined threshold level. In some embodiments, the gas is supplied at a plurality of locations along the circulation path of the bioreactor system.

In some embodiments, the organic carbon is supplied at a plurality of locations along the circulation path of the bioreactor system. In some embodiments, the light is supplied at a plurality of locations along the circulation path of the bioreactor system. In some embodiments, the method further comprises harvesting the mixotrophic microorganisms from the bioreactor system.

In some embodiments, controlling the at least one light source comprises adjusting at least one selected from the group consisting of: amount of light, photoperiod of light, flashing or pulsing period of light, wavelength of light, and intensity of light. In some embodiments, at least one of an amount and intensity of light is increased when the detected dissolved oxygen level is below the predetermined threshold level. In some embodiments, at least one of an amount and intensity of light is decreased when the detected dissolved oxygen level is above the predetermined threshold level.

In some embodiments, the organic carbon comprises at least one selected from the group consisting of: acetate, acetic acid, ammonium linoleate, arabinose, arginine, aspartic acid, butyric acid, cellulose, citric acid, ethanol, fructose, fatty acids, galactose, glucose, glycerol, glycine, lactic acid, lactose, maleic acid, maltose, mannose, methanol, molasses, peptone, plant based hydrolyzate, proline, propionic acid, ribose, sacchrose, partial or complete hydrolysates of starch, sucrose, tartaric, TCA-cycle organic acids, thin stillage, urea, industrial waste solutions, and yeast extract.

In some embodiments of the invention, a method of growing a mixotrophic culture of microorganisms comprises: circulating a culture of mixotrophic microorganisms in an aqueous culture medium through a circulation path of a bioreactor system comprising at least one light source for supplying the light, at least one carbon dioxide supply device for supplying carbon dioxide, and at least one organic carbon supply device for supplying organic carbon to the culture of mixotrophic microorganisms; detecting dissolved oxygen and pH levels of the culture of mixotrophic microorganisms with sensors; transmitting data from the sensors to a programmable logic control system (PLC), wherein the PLC controls the supply of carbon dioxide and organic carbon to the culture of mixotrophic microorganisms; and continuously adjusting the supply of at least one selected from the group consisting of carbon dioxide and organic carbon to the culture of mixotrophic microorganisms through the PLC based on the sensor data to maintain the at least one selected from the group consisting of the pH and dissolved oxygen levels of the culture at a predetermined threshold level.

In some embodiments, the supply of carbon dioxide and organic carbon is decreased when the pH level is below the predetermined threshold level. In some embodiments, the supply of carbon dioxide is increased when the pH level is above the predetermined threshold level and the dissolved oxygen level is below the predetermined threshold level. In some embodiments, the supply of organic carbon is increased when the pH level is above the predetermined threshold level and the dissolved oxygen level is above the predetermined threshold level. In some embodiments, the organic carbon is acetic acid.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a flow chart for controlling the balance of gases in a culture of microorganisms.

FIG. 2 shows the monitored dissolved oxygen and pH levels of culture of microorganisms in a balanced mixotrophy method.

FIG. 3 shows the monitored dissolved oxygen and pH levels of culture of microorganisms in a balanced mixotrophy method.

DESCRIPTION OF THE INVENTION Definitions

The term “microorganism” refers to microscopic organisms such as microalgae and cyanobacteria. Microalgae include microscopic multi-cellular plants (e.g. duckweed), photosynthetic microorganisms, heterotrophic microorganisms, diatoms, dinoflagelattes, and unicellular algae.

The terms “microbiological culture”, “microbial culture”, or “microorganism culture” refer to a method or system for multiplying microorganisms through reproduction in a predetermined culture medium, including under controlled laboratory conditions. Microbiological cultures, microbial cultures, and microorganism cultures are used to multiply the organism, to determine the type of organism, or the abundance of the organism in the sample being tested. In liquid culture medium, the term microbiological, microbial, or microorganism culture generally refers to the entire liquid medium and the microorganisms in the liquid medium regardless of the vessel in which the culture resides. A liquid medium is often referred to as “media”, “culture medium”, or “culture media”. The act of culturing is generally referred to as “culturing microorganisms” when emphasis is on plural microorganisms. The act of culturing is generally referred to as “culturing a microorganism” when importance is placed on a species or genus of microorganism. Microorganism culture is used synonymously with culture of microorganisms.

Microorganisms that may grow in mixotrophic culture conditions include microalgae, diatoms, and cyanobacteria. Non-limiting examples of mixotrophic microorganisms may comprise organisms of the genera: Agmenellum, Amphora, Anabaena, Anacystis, Apistonema, Pleurochyrsis, Arthrospira (Spirulina), Botryococcus, Brachiomonas, Chlamydomonas, Chlorella, Chloroccum, Cruciplacolithus, Cylindrotheca, Coenochloris, Cyanophora, Cyclotella, Dunaliella, Emiliania, Euglena, Extubocellulus, Fragilaria, Galdieria, Goniotrichium, Haematococcus, Halochlorella, Isochyrsis, Leptocylindrus, Micractinium, Melosira, Monodus, Nostoc, Nannochloris, Nannochloropsis, Navicula, Neospongiococcum, Nitzschia, Odontella, Ochromonas, Ochrosphaera, Pavlova, Picochlorum, Phaeodactylum, Pleurochyrsis, Porphyridium, Poteriochromonas, Prymnesium, Rhodomonas, Scenedesmus, Skeletonema, Spumella, Stauroneis, Stichococcus, Auxenochlorella, Cheatoceros, Neochloris, Ocromonas, Porphiridium, Synechococcus, Synechocystis, Tetraselmis, Thraustochytrids, Thalassiosira, and species thereof.

The organic carbon sources suitable for growing a microorganism mixotrophically or heterotrophically may comprise: acetate, acetic acid, ammonium linoleate, arabinose, arginine, aspartic acid, butyric acid, cellulose, citric acid, ethanol, fructose, fatty acids, galactose, glucose, glycerol, glycine, lactic acid, lactose, maleic acid, maltose, mannose, methanol, molasses, peptone, plant based hydrolyzate, proline, propionic acid, ribose, sacchrose, partial or complete hydrolysates of starch, sucrose, tartaric, TCA-cycle organic acids, thin stillage, urea, industrial waste solutions, yeast extract, and combinations thereof. The organic carbon source may comprise any single source, combination of sources, and dilutions of single sources or combinations of sources.

The terms “mixotrophic” and “mixotrophy” refer to culture conditions in which light, organic carbon, and inorganic carbon (e.g., carbon dioxide, carbonate, bi-carbonate) may be applied to a culture of microorganisms. Microorganisms capable of growing in mixotrophic conditions have the metabolic profile of both phototrophic and heterotrophic microorganisms, and may use both light and organic carbon as energy sources, as well as both inorganic carbon and organic carbon as carbon sources. A mixotrophic microorganism may be using light, inorganic carbon, and organic carbon through the phototrophic and heterotrophic metabolisms simultaneously or may switch between the utilization of each metabolism. A microorganism in mixotrophic culture conditions may be a net oxygen or carbon dioxide producer depending on the energy source and carbon source utilized by the microorganism. Microorganisms capable of mixotrophic growth comprise microorganisms with the natural metabolism and ability to grow in mixotrophic conditions, as well as microorganisms which obtain the metabolism and ability through modification of cells by way of methods such as mutagenesis or genetic engineering.

The terms “phototrophic”, “phototrophy”, “photoautotrophy”, “photoautotrophic”, and “autotroph” refer to culture conditions in which light and inorganic carbon (e.g., carbon dioxide, carbonate, bi-carbonate) may be applied to a culture of microorganisms. Microorganisms capable of growing in phototrophic conditions may use light as an energy source and inorganic carbon (e.g., carbon dioxide) as a carbon source. A microorganism in phototrophic conditions may produce oxygen.

The terms “heterotrophic” and “heterotrophy” refer to culture conditions in which organic carbon may be applied to a culture of microorganisms in the absence of light. Microorganisms capable of growing in heterotrophic conditions may use organic carbon as both an energy source and as a carbon source. A microorganism in heterotrophic conditions may produce carbon dioxide.

The term “axenic” describes a culture of an organism that is entirely free of all other “contaminating” organisms (i.e., organisms that are detrimental to the health of the microalgae or cyanobacteria culture). Throughout the specification, axenic refers to a culture that when inoculated in an agar plate with bacterial basal medium, does not form any colonies other than the microorganism of interest. Axenic describes cultures not contaminated by or associated with any other living organisms such as but not limited to bacteria, cyanobacteria, microalgae and/or fungi. Axenic is usually used in reference to pure cultures of microorganisms that are completely free of the presence of other different organisms. An axenic culture of microalgae or cyanobacteria is completely free from other different organisms.

Bacteria that may be present in cultures of microalgae and cyanobacteria comprise, but are not limited to: Achromobacter sp., Acidovorax sp., Acinetobacter sp., Aeromonas sp., Agrobacterium sp., Alteromonas sp., Ancylobacter sp., Aquaspirillum sp., Azospirillum sp., Azotobacter sp., Bacillus sp., Bergeyella sp., Brevundimonas sp., Brochothrix sp., Brumimicrobium sp., Burkholderia sp., Caulobacter sp., Cellulomonas sp., Chryseobacterium sp., Curtobacterium sp., Delftia sp., Empedobacter sp., Enterobacter sp., Escherichia sp., Flavobacterium sp., Gemmatimonas sp., Halomonas sp., Hydrogenophaga sp., Janthinobacterium sp., Lactobacillus sp., Marinobacter sp., Massilia sp., Microbacterium sp., Myroides sp., Pantoea sp., Paracoccus sp., Pedobacter sp., Phaeobacter sp., Phyllobacterium sp., Pseudoalteromonas sp., Pseudomonas sp., Rahnella sp., Ralstonia sp., Rhizobium sp., Rhodococcus sp., Roseomonas sp., Sphingobacterium sp., Sphingomoas sp., Staphylococcus sp., Stenotrophomonas sp., Vibrio sp., and Zobelliae sp.

Bacteria that have a negative or harmful effect on the microalgae and cyanobacteria may be designated as contaminating bacteria. The bacteria that may have a negative or harmful effect on microalgae or cyanobacteria in a culture comprise, but are not limited to: Achromobacter sp., Acidovorax sp., Aeromonas sp., Agrobacterium sp., Alteromonas sp., Aquaspirillum sp., Azospirillum sp., Azotobacter sp., Bergeyella sp., Brochothrix sp., Brumimicrobium sp., Burkholderia sp., Caulobacter sp., Cellulomonas sp., Chryseobacterium sp., Curtobacterium sp., Delftia sp., Empedobacter sp., Enterobacter sp., Escherichia sp., Flavobacterium sp., Marinobacter sp., Microbacterium sp., Myroides sp., Paracoccus sp., Pedobacter sp., Phaeobacter sp., Pseudoalteromonas sp., Pseudomonas sp., Rahnella sp., Ralstonia sp., Rhizobium sp., Rhodococcus sp., Roseomonas sp., Staphylococcus sp., Stenotrophomonas sp., Vibrio sp., Zobelliae sp. and other bacteria which share similar characteristics.

The bacteria that may have a neutral or beneficial effect on microalgae or cyanobacteria in a culture comprise, but are not limited to: Acidovorax sp., Acinetobacter sp., Aeromonas sp., Agrobacterium sp., Alteromonas sp., Ancylobacter sp., Azospirillum sp., Azotobacter sp., Bacillus sp., Brevundimonas sp., Brumimicrobium sp., Burkholderia sp., Caulobacter sp., Cellulomonas sp., Delftia sp., Empedobacter sp., Gemmatimonas sp., Halomonas sp., Hydrogenophaga sp., Janthinobacterium sp., Lactobacillus sp., Marinobacter sp., Pantoea sp., Paracoccus sp., Phaeobacter sp., Phyllobacterium sp., Pseudoalteromonas sp., Pseudomonas sp., Rhizobium sp., Sphingomoas sp., Zobelliae sp. and other bacteria which share similar characteristics. While bacteria in a particular genus generally have the same characteristics, it is recognized that a genus of bacteria with the majority of species generally identified as harmful to microalgae or cyanobacteria may also include a particular species within the genus which is neutral or beneficial to a specific culture of microalgae or cyanobacteria, and vice versa. For example, many species of Pseudomonas have been observed to be harmful to microalgae, however literature has described certain species of Pseudomonas with anti-fungal functionality which may be beneficial to a culture of microalgae or cyanobacteria.

The term “pH auxostat” refers to the microbial cultivation technique that couples the addition of fresh medium (e.g., medium containing organic carbon or acetic acid) to pH control. As the pH drifts from a given set point, fresh medium is added to bring the pH back to the set point. The rate of pH change is often an excellent indication of growth and meets the requirements as a growth-dependent parameter. The feed will keep the residual nutrient concentration in balance with the buffering capacity of the medium. The pH set point may be changed depending on the microorganisms present in the culture at the time. The microorganisms present may be driven by the location and season where the bioreactor is operated and how close the cultures are positioned to other contamination sources (e.g., other farms, agriculture, ocean, lake, river, waste water). The rate of medium addition is determined by the buffering capacity and the feed concentration of the limiting nutrient and not directly by the set point (pH) as in a traditional auxostat. The pH auxostat is robust but controls nutrient concentration indirectly. The pH level represents the summation of the production of different ionic species and ion release during carbon and nutrient uptake. Therefore the pH level can move either up or down as a function of growth of the microorganisms. The most common situation is pH depression caused by organic acid production and ammonium uptake. However, for microorganisms growing on protein or amino acid-rich media, the pH level will rise with growth because of the release of excess ammonia.

The term “harvesting” refers to removing the culture of microorganisms from the culturing vessel and/or separating the microorganisms from the culture medium. Harvesting of microorganisms may be conducted by any method known in the art such as, but not limited to, skimming, draining, dissolved gas flotation, foam fractionation, centrifugation, filtration, sedimentation, chemical flocculation, and electro-dewatering.

Balanced Mixotrophy Overview

A bioreactor system operating in a state which continuously balances the composition of gases may provide numerous benefits to a culture of mixotrophic microorganisms, such as microalgae and cyanobacteria. By balancing the oxygen and carbon dioxide levels in a mixotrophic culture continuously, the gas exchange may be optimized to: increase the efficiency of organic carbon use in the microorganism culture; reduce the amount of foam in the bioreactor system; reduce the amount of gases lost in the culture by bubbling out of the aqueous medium before the gases are utilized by the microorganisms; reduce the amount of gas supplied by external sources; and reduce the microorganisms growth inhibiting effects caused by toxic oversaturation of gases in the culture medium. Through the optimization provided by a mixotrophic culturing method continuously balancing the gas composition, the combined effects may result in an increase in efficiency in gas and organic carbon utilization. Increased efficiency in gas and organic carbon utilization may reduce overall costs of the culturing process and improve growth or culture health.

Continually balancing the gas composition of a mixotrophic culture may also allow the system to exchange gases as the culture circulates in the bioreactor system without the use of a de-gassing tank (i.e., tank where excess gases are removed from a culture medium), or substantially reduce the amount of foam created in the de-gassing tank and bioreactor system. The elimination of a de-gassing tank may reduce the cost of the bioreactor system and simplify the circulation path. The reduction of foam may reduce the level of contaminating organisms in the culture. Continuously balancing the gas composition of a mixotrophic culture may also allow for the reduction in the gas supplied by an external source, or the elimination of the external gas input, with the microorganisms solely using the oxygen and carbon dioxide produced by the phototrophic and heterotrophic metabolisms. Additionally, continuously balancing the gas composition and pH of a culture will allow the bioreactor system to have a larger volume and longer circulation path for the aqueous culture. Through the supply of light, organic carbon, and optionally some carbon dioxide to continuously adjust the culture conditions, the mixotrophic microorganisms in the culture may shift between the use of phototrophic and heterotrophic metabolisms to maintain a balanced gas composition and pH level.

A method of continuously balancing the gas composition of a culture of a single species of mixotrophic microorganisms may be distinguished from other culturing methods which co-culture two different microorganisms in a symbiotic relationship with the co-culture comprising one microorganism being phototrophic and one microorganism being heterotrophic. A co-culture of different microorganisms introduces the complexity of managing two different organisms which may differ in a variety of aspects such as growth rate, nutrient consumption rate, shear sensitivity, and preferred culture parameters (e.g., pH, temperature, salinity, light exposure). Also, the fluctuation of any of these aspects in a symbiotic co-culture may cause the balance between the microorganisms to be upset and the decline of both microorganism species.

For example, if the conditions in the co-culture change to favor the phototrophic microorganisms, the proliferation of the phototrophic microorganisms may deplete resources for the heterotrophic microorganisms and produce a toxic level of saturated oxygen which inhibits the functions of the phototrophic microorganisms. In a method of continuously balancing the gas composition of a culture of a single species of mixotrophic microorganism, the culture conditions only have to be tailored for a single microorganism type, which allows the mixotrophic microorganisms to better cope with fluctuations in the culture conditions and maximize the available inputs for optimal growth or product accumulation.

A method of continuously balancing the gas composition of a culture of a single species of mixotrophic microorganism may also be distinguished from other methods in which a culture of microorganisms are cultured in defined sequential periods of phototrophic conditions and heterotrophic conditions. When a culture of microorganisms is cultured in defined sequential stages of phototrophy and heterotrophy, the system still has to supply gases (e.g., oxygen, carbon dioxide) from an external source for consumption by the microorganisms. Also, the sequential culturing of the microorganisms in different culture conditions may cause the microorganism to go into a lipid maturation or other product accumulation stage when in heterotrophic conditions for a sustained period of time due to the stress of changing culture conditions. In a method of continuously balancing the gas composition of a culture of a single species of mixotrophic microorganism, the amount of gases supplied by external sources may be reduced or eliminated by consuming gases produced by the mixotrophic microorganisms. Continually balancing the gas composition may also maintain a longer growth phase (i.e., cell division to increase the cell count of the culture) while utilizing both the phototrophic and heterotrophic metabolisms.

System

In some embodiments, a bioreactor system for growing mixotrophic microorganisms in an aqueous culture medium may comprise a culturing vessel, at least one light source, at least one organic carbon supply device, at least one sensor, and a programmable logic control system (PLC). In some embodiments, the bioreactor system may further comprise at least one gas supply device such as, but not limited to, a carbon dioxide, oxygen, nitrogen, or air injection or bubbling device as known in the art. In some embodiments, the system may comprise additional nutrient supply devices as known in the art for nutrients such as, but not limited to nitrates, micronutrients, macronutrients, phosphates, and trace metals. The culturing vessel may comprise any suitable open or closed vessel capable of axenic or non-axenic microorganism culture growth such as, but not limited to, a tank, a column bioreactor, a trough, a tubular bioreactor, a flat panel bioreactor, a bag bioreactor, a pond, and a raceway pond. In some embodiments, the bioreactor system may further comprise a heat exchanger for adjusting the temperature of the culture such as but not limited to, heating or cooling coils, or a shell and tube heat exchanger.

In some embodiments, the light source may comprise natural light (e.g., sunlight), at least one artificial lighting device, or combinations thereof. The at least one artificial lighting device may comprise any artificial lighting device capable of supplying light to a culture of microorganisms such as, but not limited to, fluorescent tubes, light emitting diodes (LED), micro LEDs, high pressure sodium lamps, high intensity discharge lamps, neon lamps, metal vapor lamps, halogen lamps, sulfur plasma lamps, and incandescent bulbs. In some embodiments, the at least one artificial lighting device may be selected or tuned to provide light of a particular wavelength spectrum or combination of spectrums such as, but not limited to, violet (about 380-450 nm), blue (about 450-495 nm), green (about 495-570 nm), yellow (about 570-590 nm), orange (about 590-620 nm), red (about 620-750 nm), and far red (about 700-800 nm), infrared (IR) (about 1,000-20,000 nm) and ultraviolet (UV) (about 10-400 nm).

In some embodiments, the supply of light may be continuous, discontinuous, flashing, or pulsing to create any desired light/dark cycle. In some embodiments, the intensity of light supplied by the at least one artificial lighting device may comprise a constant intensity or varying intensities. In some embodiments, the lighting may comprise narrow band LED lighting with intensities from 100-10,000 μmol photon/m² s, and wavelengths or combinations of wavelengths between 300-800 nm. In some embodiments, the natural light may be filtered or partially blocked before reaching the culture of microorganisms to control the amount, intensity, or wavelength of light.

The at least one sensor may comprise at least one selected from the group consisting of optical density, turbidity, dissolved oxygen, dissolved carbon dioxide, temperature, light (i.e., photodetector), photosynthetically active radiation (PAR), and pH sensors. The at least one sensor may monitor and detect conditions in the mixotrophic culture and transmit the data to the PLC. The PLC system may comprise any controller and computer system known in the art capable of controlling actuators and valves. The PLC may use the detected culture data to control at least one from the group consisting of: the organic carbon supply device, at least one artificial lighting device, a light filter or shade, additional nutrient supply device, and a gas supply device. In the alternative, an operator may manually control the supply of organic carbon, light, and/or gases based on the detected parameters and sensor data. In other embodiments, the system may be run stoichiometrically without any sensors based off of the estimated culture density, amount of light supplied, the amount of gas supplied (e.g., oxygen, carbon dioxide), and/or the amount and type of organic carbon source supplied.

The at least one organic carbon supply device may comprise a metering, dosing, or injection device as is known in the art. In some embodiments, the organic carbon source may be in a concentrated form. In some embodiments, the organic carbon source may be in a diluted form. In some embodiments, the organic carbon source may be a combination of at least two different organic carbon sources.

In some embodiments, the at least one organic carbon supply, at least one gas supply device, and/or at least one artificial lighting device may comprise a plurality of devices strategically located along the circulation path of the aqueous culture through the bioreactor system. With multiple locations to add light, organic carbon and/or gases, the bioreactor system may maintain more finely tuned control over the gas exchange and pH within the culture as compared to a bioreactor system with a single supply point of light, organic carbon and/or gases.

The aqueous culture medium may be circulated through the bioreactor system with a paddle wheel, pump, jets, thrusters, gravity, or any other known means. The velocity at which the aqueous culture medium is circulated may be adjusted to provide a desired level of turbulence in the circulating aqueous culture medium and residence time in the bioreactor system.

Method

In one non-limiting exemplary method, a culture of mixotrophic microorganisms, such as microalgae and cyanobacteria, in an aqueous culture medium may be circulated through a circulation path of a bioreactor system comprising at least one light source for supplying light, at least one organic carbon supply device for supplying organic, and at least one sensor for detecting a parameter of the culture. The aqueous culture medium may comprise water and micronutrients (e.g., salts, trace metals, nitrates, phosphates) in a formulation specific for the microorganism species. During the circulation of the culture, at least one culture parameter from the group consisting of the optical density, turbidity, pH, dissolved oxygen, and dissolved carbon dioxide levels of a culture of mixotrophic microorganisms in an aqueous medium may be detected by the at least one sensor. The at least one culture parameter detected by the at least one sensor may be transmitted to a programmable logic control system (PLC), wherein the PLC controls the supply of at least one from the group consisting of light and organic carbon to the culture through the at least one light source and at least one organic carbon supply device. The PLC may continuously adjust the supply of at least one from the group consisting of light and organic carbon to the culture to maintain the at least one detected culture parameter at a predetermined threshold level. Maintenance of the culture parameters within a range of the predetermined threshold levels may reduce or eliminate the need for the external supply of gases, and optimize the production and consumption of oxygen and carbon dioxide by the mixotrophic microorganisms to achieve a balanced gas composition.

In some embodiments, the bioreactor system may further comprise at least one gas supply device which may also be controlled by the PLC to maintain the at least one detected culture parameter at a predetermined threshold level. In some embodiments, the gas may comprise carbon dioxide, oxygen, nitrogen, air, or combinations thereof. In some embodiments, no gas is injected or supplied. In some embodiments, the light may be controlled by controlling a filter or shade which partially blocks natural light to reduce the amount, intensity, or wavelength spectrum of the light. In some embodiments, the supply of light comprises controlling at least one aspect such as the amount of light, photoperiod of light, flashing or pulsing period of light, wavelength range of light, and intensity of light.

In some embodiments, the culture of mixotrophic microorganisms may be axenic and maintained in axenic conditions within the bioreactor system. In some embodiments, the culture of mixotrophic microorganisms may be non-axenic and is not maintained in axenic conditions within the bioreactor system. In some embodiments, the pH predetermined threshold level or set point may be between 6 and 9, or in other embodiments between 1 and 5. In some embodiments, the dissolved oxygen level predetermined threshold level or set point may be between about 0.1 mg O₂/L to about 30 mg O₂/L depending on bacterial population and species, as well as mixotrophic microorganism population and species. In further embodiments, the predetermined threshold level of dissolved oxygen may be between 1 mg O₂/L and 6 mg O₂/L. In some embodiments, the culture may balance the oxygen and carbon dioxide levels at the saturation point of the culture medium. In some embodiments, the predetermined threshold level of dissolved oxygen may be 25-200% of the saturation level of the aqueous culture medium. The predetermined threshold level of carbon dioxide may comprise 25-500% of the saturation level of the aqueous culture medium.

In some embodiments, the organic carbon, gas, or light may be supplied at multiple locations along the circulation path of the aqueous culture of mixotrophic microorganisms. The locations of organic carbon, gas, or light supply may be chosen strategically for a plurality of reasons such as, but not limited to the growth characteristics of the culture (e.g., growth rate, nutrient consumption rate). The organic carbon source may also be supplied at locations for continuous consumption by a culture of a determined density and traveling at a determined velocity so that the residual organic carbon in the aqueous culture medium is sustained at a low level (50-1,000 ppm) to limit the feed sources available to contaminating organisms (e.g., fungi, predators, bacteria).

In some embodiments, the organic carbon source may be supplied to the culture of mixotrophic microorganisms when the dissolved oxygen level is above the predetermined threshold level to induce an increase in the consumption of oxygen through the heterotrophic metabolism of the microorganisms. In some embodiments, the organic carbon source may be supplied to the culture of mixotrophic microorganisms to maintain the pH level in a desired pH range. In some embodiments, an organic carbon source with an acidic pH may be supplied to the culture of mixotrophic microorganisms when the pH is above the predetermined threshold level to decrease the pH level to within a desired pH range. In some embodiments, an organic carbon with a basic pH may be supplied to the culture of mixotrophic microorganisms when the pH is below the predetermined threshold level to increase the pH level to within a desired pH range. In some embodiments, the organic carbon source may be supplied to the culture of mixotrophic microorganisms when the dissolved carbon dioxide level is below the predetermined threshold level to induce an increase in the production of carbon dioxide through the heterotrophic metabolism of the microorganisms. In some embodiments, the amount of organic carbon supplied may be limited to an amount that may be readily assimilated by the primary mixotrophic microorganisms of the culture, thus reducing the residual organic carbon in the system available to the contaminating organisms in the culture.

In some embodiments, carbon dioxide may be supplied to the culture of mixotrophic microorganisms when the dissolved oxygen level is below the predetermined threshold level to induce an increase in the production of oxygen through the phototrophic metabolism of the microorganisms. In some embodiments, carbon dioxide may be supplied to the culture of mixotrophic microorganisms when the pH is above the predetermined threshold level to decrease the pH level to within a desired pH range.

In some embodiments, the amount, intensity, and/or specific wavelength spectrum of light may be adjusted if the dissolved oxygen, dissolved carbon dioxide, or pH level is outside of an acceptable range of the predetermined threshold level. The amount, intensity, and/or specific wavelength of light may be adjusted to increase the production of oxygen through the photosynthetic metabolism of the mixotrophic microorganism, such as by increasing the amount of light and/or switching to primarily the red wavelength spectrum with some light in the blue wavelength spectrum. In some embodiments, the amount of light may be adjusted by changing between pulsed or flashing light and continuous light. The amount, intensity, and/or specific wavelength of light may be adjusted to decrease the production of oxygen through the photosynthetic metabolism of the mixotrophic microorganism, such as by decreasing the amount of light, intensity of light, and/or switching to a wavelength spectrum other than red. In some embodiments, at least one of an amount and intensity of light may be increased when the detected dissolved oxygen level is below the predetermined threshold level. In some embodiments, at least one of an amount and intensity of light may be decreased when the detected dissolved oxygen level is above the predetermined threshold level.

When the culture density increases, as measured by optical density or turbidity, the demand for oxygen and carbon dioxide may increase and require the culture inputs to stimulate more metabolic activity to maintain a balanced gas composition. In some embodiments, at least one of an amount and intensity of light may be increased when the detected optical density or turbidity is above a predetermined threshold level. In some embodiments, the organic carbon supply may be increased when the detected optical density or turbidity is above a predetermined threshold level. In some embodiments, the supply of gas may be increased when the detected optical density or turbidity is above a predetermined threshold level.

In some embodiments, the culture of mixotrophic microorganisms may comprise a transition, turbulent, irregular or regular chaotic flow as the culture of microorganisms circulates in the bioreactor system. In some embodiments, the turbulent or other non-laminar flow may be provided by the velocity of the circulating culture medium, a mechanical mixing device, baffles on the surface of the culturing vessel, the pressure at which carbon dioxide or organic carbon is injected into the culture medium, and combinations thereof. In some embodiments, temperature may be monitored and adjusted within the culture by a heat exchanger such as, but not limited to, heating or cooling coils, or a shell and tube heat exchanger. In some embodiments, pressure within a closed bioreactor system may be monitored and adjusted within the bioreactor system.

In some embodiments, the balanced mixotrophy method of culturing mixotrophic microorganisms may be used to produce microorganism biomass for harvest. The microorganisms may be harvested by any known means such as, but not limited to, filtration, centrifugation, dissolved air flotation, foam fractionation, the application of electrical fields, the application of acoustic energy, and combination thereof. In some embodiments, the balanced mixotrophy method of culturing mixotrophic microorganisms may be used as a de-eutrophication method to remove nutrients (e.g., nitrates, phosphates) from a fluid stream.

In some embodiments, the bioreactor system may be run at a higher pressure and/or have a reservoir exposed to higher pressure to increase the saturation of the targeted gas thus enhancing the oxygen and/or carbon dioxide produced and consumed by the mixotrophic microorganisms. The increase in pressure within the bioreactor system increases the solubility of the gasses in solution, thus increasing the efficiency of oxygen/carbon dioxide utilization by the mixotrophic microorganisms.

In some embodiments, the pH, dissolved oxygen level, and carbon dioxide level of the culture in the bioreactor system may be operated in a steady state for at least one or all parameters. In some embodiments, the pH, dissolved oxygen level, and carbon dioxide level of the culture in the bioreactor system may be operated in a steady state for some parameters while at least one parameter fluctuates between ranged set points. For example, in one non-limiting embodiment the pH may be controlled through a set point (i.e., predetermined threshold level) on a feedback control loop while the dissolved oxygen level may be controlled and allowed to rise to the threshold of the mixotrophic microorganisms' tolerance level (e.g., a species of Chlorella has been observed to have a tolerance level of approximately 12 mg O₂/L but this value will be species specific). When the dissolved oxygen level reaches the set point (i.e., predetermined threshold level), the organic carbon supply may be increased to allow the mixotrophic microorganisms to consume the oxygen through the heterotrophic metabolism and reduce the dissolved oxygen level of the culture (ranging from saturation to 0 mg/L).

In some embodiments, the bioreactor system may be operated using methods of continuously balancing the gas composition in mixotrophic culture conditions and the mixotrophic microorganisms may be harvested for protein, lipid, pigment, carbohydrate, or polysaccharide products extracted or separated in downstream processing. In some embodiments, the light may be adjusted to stimulate pigment production during the balanced mixotrophy method to simultaneously increase targeted pigment (e.g., chlorophylls, carotenoids) during the growth phase of the microorganism culture.

Example 1

The flow chart in FIG. 1 describes a non-limiting exemplary embodiment implementing the controls for a balanced mixotrophy method which continually balances the gas composition of a culture of mixotrophic microorganisms. To begin the method, the dissolved oxygen (DO) and pH levels of the mixotrophic microorganism culture are detected. If the detected pH level is below the predetermined threshold level or set point, the carbon dioxide and organic carbon (e.g., acetic acid) supply is stopped or reduced. If the detected pH level is above the predetermined threshold level or set point, a determination is made as to whether the DO level is above or below the predetermined threshold level or set point. If the detected DO level is below the predetermined threshold level or set point, the carbon dioxide supply is started or increased to induce use of the phototrophic metabolism in the mixotrophic microorganism and the production of oxygen. If the detected DO level is above the predetermined threshold level or set point, the organic carbon (e.g., acetic acid) supply is started or increased to induce use of the heterotrophic metabolism in the microorganism and the production of carbon dioxide. The method may then loop to the beginning where DO and pH levels are measured, and the process steps are repeated. The frequency of measurements may be done at any reasonable time period for the sensors, including multiple times per hour or minute.

Example 2

A balanced mixotrophy method for continuously balancing the gas composition in a culture of mixotrophic microorganisms was tested using a tubular bioreactor system with a working volume of 400 L. The tubular bioreactor system comprised pump circulation through stacked horizontal tubes joined by U-shaped tubular joints, a de-gassing tank, with a light source consisting of arrays of fluorescent tubes, an organic carbon supply device dosing acetic acid, a carbon dioxide injection system, pH sensors, dissolved oxygen (DO) sensors, and a programmable logic control system (PLC). It is of note that the balanced mixotrophy method test did not use any injection or sparging of air into the bioreactor system.

The mixotrophic microorganism culture comprised a species of Chlorella in a BG-11 aqueous culture medium. The mixotrophic microorganism culture was continuously circulated through the tubular bioreactor system with sensors monitoring the pH and DO levels. The culture temperature was maintained within +/−5° C. of room temperature (approximately 25° C.). Fluorescent light was provided to the tubular bioreactor system at approximately 335 μmol photon/m² s in a 24 hour continuous photoperiod. The control logic for the acetic acid and carbon dioxide supply devices was the same as outlined in Example 1 and FIG. 1, with the predetermined threshold levels (i.e., set points) at 7.5 for pH and 4 mg O₂/L for DO. The DO and pH sensors detected the pH in the culture circulation at three points in the bioreactor system: before the de-gas tank, after the de-gas tank, and after the pump. The control logic of the PLC dosed acetic acid when the detected pH level was above the predetermined threshold level (i.e., 7.5) and the detected DO level was above the predetermined threshold level (i.e., 4 mg O₂/L). The control logic of the PLC dosed carbon dioxide when the detected pH level was above the predetermined threshold level (i.e., 7.5) and the detected DO level was below the predetermined threshold level (i.e., 4 mg O₂/L).

In tests conducted prior to the balanced mixotrophy method test, the test runs phototrophically culturing of Chlorella in the tubular bioreactor system with air sparging and no acetic acid created noticeable quantities of foam in the de-gas tank and were observed to operate at a high DO level (i.e., above 4 mg O₂/L). The high DO level in the prior phototrophic tests was controlled with N₂ sparging in various levels of concentration with the air.

During the balanced mixotrophy method test, air sparging was not utilized and a significant reduction in the amount of foam in the tubular bioreactor system was observed compared to test runs of the tubular bioreactor systems culturing Chlorella phototrophically with air sparging. As shown in FIG. 2, it was also observed that the DO level was able to be maintained in an acceptable range (+/−2 mg O₂/L) of the 4 mg O₂/L predetermined threshold level without the sparing of air. FIG. 2 also demonstrates that the pH level was able to be maintained within an acceptable range of the 7.5 (+/−0.1) predetermined threshold level using the acetic acid and carbon dioxide dosing.

Growth rates in prior phototrophic growth tests averaged 0.5 to 0.6 g/L per day when the tubular bioreactor system cultured microalgae phototrophically with air sparging. Growth rates for the first three days of the balanced mixotrophy method test without air sparging comprised 1.1 g/L, 1.2 g/L, and 1.25 g/L per day, with an average of 1.13 g/L per day over a six day period. The average growth rate of 1.13 g/L per day for the balanced mixotrophy tests was approximately two times the average of the growth rates obtained in the prior phototrophic growth tests with the tubular bioreactor system.

The growth rate data of 1.1 to 1.25 g/L per day was found to be consistent with the theorized growth rates if the mixotrophic microalgae are promptly consuming gas molecules (i.e., oxygen, carbon dioxide) created within the culture by the metabolisms of the microalgae. By using the balanced mixotrophy method, it was observed that the balanced gas composition of the culture reduced the foam to near imperceptible levels, which may reduce the potential for contamination and growth inhibition associated with the foam. The acetic acid supplied and consumed during the six day period of the balanced mixotrophy method averaged 5.36 Liters/Day, and resulted in an acetic acid efficiency of 1.89 g of acetic acid/g of microalgae biomass growth. The acetic acid efficiency of 1.89 g of acetic acid/g of microalgae biomass growth from the balanced mixotrophy method was better than the average of 3.5 g of acetic acid/g of microalgae biomass growth for previous tests with conventional mixotrophy methods comprising both air sparging and carbon dioxide injection.

As shown in FIG. 3, it was also observed that the pH level was able to be maintained within an acceptable range (+/−0.25) of the 7.5 predetermined threshold level using the acetic acid and carbon dioxide dosing of the balanced mixotrophy method when the DO predetermined threshold level (i.e., set point) was raised from 4 mg O₂/L to 10 mg O₂/L. When the DO predetermined threshold level was changed, the DO level quickly rose to the new predetermined threshold level of 10 mg O₂/L and was maintained within an acceptable range (+/−2 mg O₂/L) of the new predetermined threshold level.

Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims. 

What is claimed is:
 1. A method of growing a mixotrophic culture of microorganisms, comprising: a. Providing a culture of mixotrophic microorganisms in an aqueous culture medium with a supply of carbon dioxide and a supply of organic carbon comprising at least one organic carbon source selected from the group consisting of acetic acid and acetate; b. Circulating the culture of mixotrophic microorganisms through a circulation path of a bioreactor system selected from the group consisting of a tubular bioreactor and a raceway pond, wherein the culture receives the supply of carbon dioxide and the supply of organic carbon in the circulation path; c. Detecting a pH level and a dissolved oxygen level in the culture with at least one sensor disposed along the circulation path; d. Continuously adjusting the supply of carbon dioxide and organic carbon to the culture based on the detected pH level and dissolved oxygen level in the culture, wherein: i. the carbon dioxide is supplied and the organic carbon is not supplied when the detected pH level is above a predetermined threshold level and the detected dissolved oxygen level is below a predetermined threshold level; and ii. the carbon dioxide is not supplied and the organic carbon is supplied when the detected pH level is above a predetermined threshold level and the detected dissolved oxygen level is above a predetermined threshold level.
 2. The method of claim 1, wherein the culture of microorganisms comprises at least one selected from the group consisting of microalgae and cyanobacteria.
 3. The method of claim 1, wherein a composition of the dissolved oxygen and dissolved carbon dioxide is balanced at the saturation point of the aqueous culture medium.
 4. The method of claim 3, wherein the balanced composition of dissolved oxygen and dissolved carbon dioxide comprises oxygen at 25-200% of the saturation level of the aqueous culture medium and carbon dioxide at 25-500% of the saturation level of the aqueous culture medium.
 5. The method of claim 1, wherein the predetermined threshold of dissolved oxygen level is in the range of 0.1-30 mg O₂/L.
 6. The method of claim 5, wherein the predetermined threshold dissolved oxygen level is in the range of 1-6 mg O₂/L.
 7. The method of claim 1, wherein the method further comprises transmitting data from the at least one sensor to a programmable logic control system to control the supply of carbon dioxide and the supply of organic carbon.
 8. The method of claim 1, wherein culture does not receive an external supply of gas other than the selectively supplied carbon dioxide.
 9. The method of claim 8, wherein the culture produces 85% more grams of microorganism biomass growth per gram of acetic acid than a culture receiving an external supply of gases.
 10. The method of claim 1, wherein the predetermined threshold pH level is in the range of 6-9.
 11. The method of claim 1, wherein the predetermined threshold pH level is in the range of 1-5.
 12. The method of claim 1, wherein a residual organic carbon level in the culture is maintained in the range of 50-1,000 ppm.
 13. The method of claim 1, wherein at least one of the supply of carbon dioxide and supply of organic carbon is supplied at a plurality of locations along the circulation path of the bioreactor system.
 14. The method of claim 1, further comprising supplying light to the culture.
 15. The method of claim 14, wherein the supply of light is increased when the organic carbon is not supplied.
 16. The method of claim 14, wherein the supply of light is decreased when the organic carbon is supplied.
 17. The method of claim 14, wherein the supply of light comprises an intensity ranging from 100-10,000 μmol photon/m²s.
 18. The method of claim 14, wherein the supply of light comprises a wavelength range between 300-800 nm.
 19. The method of claim 14, wherein the supply of light comprises at least one of natural light and artificial light.
 20. The method of claim 14, wherein light is supplied at a plurality of locations along the circulation path of the bioreactor system. 